1. Field of the Invention
The present invention relates to a noise control system and method of operation. More specifically, the present invention relates to an active noise control system and a method which cancels a noise by generating a control sound.
2. Description of the Related Art
By generating a sound (control sound) having the same frequency and amplitude as the noise but at a 180.degree. shifted phase from the noise sounds from a loudspeaker, the noise can be canceled. Because the control sound interferes with the noise and cancels the noise in the region where the control sound is generated (usually, at a listening position). Such a noise control method is called an active noise control. Unlike conventional noiseproofing methods, the active noise control does not require a cover to the noise source (such as an engine) with noise absorption material or to block out noise with a noise insulation material. Consequently, it has the advantages of being able to make the entire sound control system lighter and smaller.
In order to effectively cancel a noise with an active noise control system, the control sound adapted to the noise must be precisely generated. A practical active noise control could not have been created until the development of appropriate algorithms and signal processing technologies. With the recent development of digital signal processing technologies, practical active noise control systems have been proposed.
FIG. 17 schematically shows an example of noise control by a conventional noise control system 700. The configuration of the conventional noise control system 700 and the noise control method using the system 700 will be described below with reference to the relevant drawings.
FIG. 17 illustrates the case of canceling a noise in the region P (noise control area) by generating a control sound from a loudspeaker 704 of the noise control system 700 when the noise travels from a noise source 701.
The noise from the noise source 701 (with the characteristic N) reaches the region P via a transfer function G. At the same time, the information of the noise source 701 is detected by a noise detector 702 (with transfer function M) of the noise control system 700. The detected noise is applied to an adaptive filter 703 ( with transfer function H) as a detection signal. The adaptive filter 703 performs an adaptive processing on the detection signal so as to produce a control signal. The control signal is radiated from a control loudspeaker 704 (with transfer function S) as a control sound, and the control sound reaches the region P via a control transfer characteristic (with transfer function C) including a loudspeaker characteristic S.
The control sound and the noise are synthesized in the region P, and if the control sound is optimized, the entire noise in principle is canceled. However, in reality, there is a residual error as shown in Expression (1), due to the fluctuations of the respective transfer characteristics or the like. EQU residual error=noise+control sound (1)
The residual error is detected by an error detector (an error microphone) 705 (with transfer function E) and applied to a multiplier 706 as an error signal e. The error signal e can be expressed by Expression (2) using transfer characteristics (transfer functions) of each system. EQU e=(N*G+N*M*H*C)*E (2)
The coefficients of the adaptive filter 703 (i.e., transfer function H) are corrected so that the error signal e is minimized. The filter coefficients are corrected with a coefficient update vector .DELTA.H given by the multiplier 706. The multiplier 706 calculates the coefficient update vector .DELTA.H based on the error signal e and a reference input X using the LMS (Least Mean Square) algorithm.
As the reference input X, instead of directly using the noise detection signal detected by the noise detector 702, the noise detection signal is used after passing through an operator 707 (with filter coefficients, i.e., transfer function C'). The transfer function C' is set by simulating the predetermined control system transfer function C. Specifically, the transfer function C' is set so as to be an inverse transfer function having the same frequency-amplitude characteristic as the transfer function C but at a 180.degree. shifted phase from the transfer function C. The frequency characteristic and the impulse response of the operator 707 is shown in FIGS. 18 and 19, respectively. By using the noise detection signal through the operator 707, the phase of the reference signal X matches that of the error signal e, and thus, the coefficient calculation of the adaptive filter 703 can be correctly converged.
The transfer function of the adaptive filter 703 is given by Expression (3) by considering the left side of Expression (2) as nearly equal to 0. EQU H.apprxeq.-G/ (M*C) (3)
By successively updating the coefficients of the adaptive filter 703, the transfer function H can be updated to follow the changes of each transfer function, and thereby the noise control effects can be maintained.
However, since a conventional noise control system as described above, has to perform a significant amount of operation processing in the multiplier 706, large hardware is required to calculate the coefficients of the adaptive filter 703. For instance, the operator 707 is composed of a digital filter having a tap length of 128 as shown in FIG. 19. This is because the lower sound range of a transfer characteristic can not be reproduced by the operator 707 if the number of the taps in the operator 707 is insufficient. Extra noise is thereby disadvantageously added or diverged. It also increases the cost of the system compared with noise absorption or noise insulation materials.
Moreover, when the control loudspeaker 704 is placed apart from the error detector 705, the following problems are caused. In such a case where an intervening object is placed between the control loudspeaker 704 and the error detector 705, or the surroundings are changed after determining the transfer function C of the control loudspeaker 704 to the error detector 705 and setting the inverse transfer function C' at the operator 707, the transfer characteristic is affected by the intervening object and develops many peaks and dips. The coefficients of the adaptive filter 703 cannot follow such a large change of the transfer characteristics and sufficient noise control effects can not be obtained.